Are all 1974 pennies made out of aluminum?

NO circulation 1974 cents are made of aluminum. All were made of bronze (copper, tin, and zinc).

You are probably thinking of the famous case of a set of experimental coins that were struck in aluminum to see if it would be a reasonable substitute for bronze. The coins turned out to be too light (for ex. they'd blow away in a breeze) so they were supposedly all melted. However some of them had been loaned to members of Congress for examination, and rumors persist that a few of the coins were never sent back for melting. However no one has ever seen any of the coins except for a few that were kept for museums.